Using generic interfaces

Question

Is there any differences between those two class declarations

1:

class MyClass <T extends Number & Comparable>

2:

class MyClass <T extends Number & Comparable<T>>

I think that there are differences. But I cannot find an example which would show differences because I don't understand it exact.

Can you show me this example?

Your first snippet uses a raw type, which you shouldn't use. — arshajii, Mar 10 '14 at 18:32
@Paul Bellorano, I read this topic. I have concrete question! — gstackoverflow, Mar 10 '14 at 18:33
I suggest you re-read it then. It addresses the question exactly. — Paul Bellora, Mar 10 '14 at 18:35
@gstackoverflow: I agree with Paul Bellora. Your question finally boils down to the question linked by him. — Bhesh Gurung, Mar 10 '14 at 18:39
You should go through the tutorial on [Java Generics](http://docs.oracle.com/javase/tutorial/extra/generics/index.html), get more familiar with the basics and the terms. And then you will probably agree with him too. — Bhesh Gurung, Mar 10 '14 at 18:44

Marco13 · Accepted Answer · 2014-03-10T18:44:21.447

5

There is a difference. The first one is using raw types, and thus, is less type-safe. For example:

This works, but should not work

class MyClass<T extends Number & Comparable>
{
    void use(T t)
    {
        String s = null;
        t.compareTo(s); // Works, but will cause a runtime error
    }
}

Whereas this does not work (because it should not work)

class MyClass<T extends Number & Comparable<T>>
{
    void use(T t)
    {
        String s = null;
        t.compareTo(s); // Compile-time error
    }
}

EDIT: Full code, as requested:

class MyClass<T extends Number & Comparable>
{
    void use(T t)
    {
        String s = "Laziness";
        t.compareTo(s); // Works, but will cause a runtime error
    }
}


public class MyClassTest
{
    public static void main(String[] args)
    {
        MyClass<Integer> m = new MyClass<Integer>();
        Integer integer = new Integer(42);
        m.use(integer);
    }
}

edited Mar 10 '14 at 18:44

answered Mar 10 '14 at 18:34

Marco13

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**This works, but should not work** excellent phrase – gstackoverflow Mar 10 '14 at 18:35
**// Works, but will cause a runtime error** can you show full code? – gstackoverflow Mar 10 '14 at 18:37
@gstackoverflow Create an instance of first `MyClass`, and invoke its `use()` method, passing an appropriate argument. This much you can do I think. – Rohit Jain Mar 10 '14 at 18:39
@gstackoverflow Added an example – Marco13 Mar 10 '14 at 18:44

Using generic interfaces

1 Answers1